Molecular Basis of Inheritance: DNA, Replication, Transcription and Translation for NEET Preparation

Understanding how genetic information is stored and expressed is essential for solving questions from Molecular Basis of Inheritance in NEET Biology. Concepts such as DNA structure, replication, transcription, translation and gene regulation are closely connected, so learning them as separate facts can make revision difficult.

Molecular Basis of Inheritance explains how DNA and RNA function as genetic material, how DNA replicates, and how genetic information is converted into proteins. It also covers the experiments that established DNA as genetic material, the genetic code, the lac operon, the Human Genome Project and DNA fingerprinting through VNTRs.

Nucleic Acids as Genetic Material

Nucleic acids are polymers made of nucleotides and are broadly classified into DNA (Deoxyribonucleic Acid) and RNA (Ribonucleic Acid).

DNA is better suited for long-term storage of genetic information because it is more stable. RNA is comparatively more reactive and less stable, but it can perform several functions, including acting as genetic material in some viruses, forming part of ribosomes, acting as a catalyst, and functioning as an adapter during protein synthesis.

Structure of a Nucleotide

Each nucleotide consists of three components:

• Nitrogenous base

• Pentose sugar

• Phosphate group

Nitrogenous bases are of two types:

• Purines: Adenine (A) and Guanine (G)

• Pyrimidines: Cytosine (C), Thymine (T), and Uracil (U)

DNA contains A, T, G, and C, whereas RNA contains A, U, G, and C.

DNA contains deoxyribose, while RNA contains ribose. The major difference is at the 2′ carbon, where ribose has an -OH group and deoxyribose has -H. The 2′-OH group contributes to the greater reactivity of RNA.

A nitrogenous base and sugar form a nucleoside. Addition of a phosphate group forms a nucleotide.

Nucleotides are joined by phosphodiester bonds to form polynucleotide chains. The chain has 5′ and 3′ polarity, and synthesis occurs in the 5′ to 3′ direction.

In double-stranded DNA, complementary base pairing occurs as:

• A pairs with T through two hydrogen bonds.

• G pairs with C through three hydrogen bonds.

Chargaff's rule states that in double-stranded DNA, A = T and G = C.

DNA Packaging

DNA is negatively charged because of its phosphate groups. In prokaryotes, DNA is present in the nucleoid and associates with positively charged proteins that help compact it.

In eukaryotes, DNA is packaged with positively charged histone proteins. Eight histone molecules form a histone octamer, consisting of two copies each of H2A, H2B, H3, and H4. DNA wraps around this octamer to form a nucleosome.

Nucleosomes connected by linker DNA produce the beads-on-a-string structure. The H1 protein acts as a linker histone and helps in further chromatin organisation.

Chromatin can occur as relatively open euchromatin or more tightly packed heterochromatin. Loosely packed DNA is generally more accessible to proteins involved in gene expression.

DNA as Genetic Material

Several experiments established DNA as the genetic material.

Griffith's Experiment

Frederick Griffith studied S and R strains of Streptococcus pneumoniae. The S strain was disease-causing, while the R strain was non-virulent. Griffith observed that characteristics of the S strain could be transferred to the R strain, suggesting the presence of a transforming principle.

Avery, MacLeod and McCarty

Avery, MacLeod and McCarty investigated the transforming principle. Their experiments showed that DNA was responsible for transformation, providing evidence that DNA is the genetic material.

Hershey-Chase Experiment

The Hershey-Chase experiment used bacteriophages to distinguish between DNA and protein. DNA was labelled with radioactive phosphorus, while protein was labelled with radioactive sulfur. After infection, blending, and centrifugation, the radioactive material associated with DNA was found with bacterial cells, while labelled protein remained outside.

This provided further evidence that DNA enters bacterial cells and carries genetic information.

DNA Replication

DNA replication is semi-conservative. The two parental strands separate, and each acts as a template for synthesis of a complementary strand. Therefore, each daughter DNA molecule contains one parental strand and one newly synthesised strand.

The semi-conservative model was demonstrated by the Meselson-Stahl experiment using E. coli and nitrogen isotopes. DNA initially labelled with heavy nitrogen was transferred to a medium containing light nitrogen. After replication, intermediate-density DNA appeared, and subsequent replication produced both intermediate and light DNA.

Machinery of DNA Replication

Replication begins at an origin of replication and produces a replication fork.

Important components include:

• Helicase: Separates DNA strands by breaking hydrogen bonds.

• Single-strand binding proteins: Keep separated DNA strands from rejoining.

• Topoisomerases: Relieve tension produced during DNA unwinding.

• RNA primer: Provides the free 3′-OH needed to begin DNA synthesis.

• DNA Polymerase III: Main enzyme involved in DNA synthesis.

• DNA Polymerase I: Removes RNA primers and replaces them with DNA.

• DNA Ligase: Joins Okazaki fragments.

Since DNA synthesis occurs only in the 5′ to 3′ direction, the leading strand is synthesised continuously, while the lagging strand is synthesised discontinuously as Okazaki fragments.

Transcription and Gene Expression

Transcription is the synthesis of RNA using DNA as a template. Only one DNA strand is used as the template for a particular gene.

A transcription unit contains:

• Promoter

• Structural gene

• Terminator

RNA synthesis occurs in the 5′ to 3′ direction, with uracil replacing thymine.

Transcription in Prokaryotes

Prokaryotes use a major RNA polymerase for transcription. The sigma factor helps RNA polymerase recognise the promoter and initiate transcription, while the rho factor is associated with termination.

Since prokaryotes lack a nuclear membrane, transcription and translation can occur in a coupled manner.

Transcription in Eukaryotes

Eukaryotes have three major nuclear RNA polymerases:

• RNA Polymerase I: Synthesises 28S, 18S, and 5.8S rRNA.

• RNA Polymerase II: Synthesises hnRNA, which is processed into mature mRNA.

• RNA Polymerase III: Synthesises tRNA, 5S rRNA, and certain small nuclear RNAs.

The initial hnRNA undergoes splicing, capping, and tailing. During splicing, introns are removed, and exons are joined. A cap is added at the 5′ end, while a poly-A tail of approximately 200–300 adenine residues is added at the 3′ end.

Genetic Code and Translation

The genetic code connects the nucleotide sequence of mRNA with the amino acid sequence of a protein. Three nucleotides form one codon. There are 64 codons, of which 61 code for amino acids and three—UAA, UAG, and UGA—are stop codons. AUG acts as the initiator codon and codes for methionine.

The genetic code is triplet, degenerate, unambiguous, comma-less, non-overlapping, and nearly universal.

Scientists including George Gamow, Har Gobind Khorana, Marshall Nirenberg, and Severo Ochoa contributed to understanding the genetic code.

Translation converts the information in mRNA into a polypeptide chain. It involves mRNA, tRNA, rRNA, ribosomes, amino acids, and initiation and release factors.

tRNA acts as an adapter molecule. Its anticodon recognises the mRNA codon, while its acceptor end carries the appropriate amino acid. It has a cloverleaf structure in two dimensions and an inverted-L structure in three dimensions.

Translation occurs through:

1. Initiation: Ribosome associates with mRNA and the initiator tRNA recognises AUG.

2. Elongation: Charged tRNAs bring amino acids, which are joined by peptide bonds.

3. Termination: A stop codon is recognised and release factors release the newly formed polypeptide.

The transcript also highlights the role of 23S rRNA as a ribozyme involved in peptide bond formation.

Regulation of Gene Expression

Cells do not express all genes at the same time or at the same level. Gene expression is regulated according to cellular, physiological, metabolic, environmental, and developmental conditions.

In a monocistronic arrangement, one mRNA generally carries information for one polypeptide. In a polycistronic arrangement, one mRNA can carry information for multiple polypeptides.

Lac Operon

The lac operon regulates genes involved in lactose metabolism in E. coli. Its structural genes are:

• lacZ: β-galactosidase

• lacY: Permease

• lacA: Transacetylase

The promoter is the binding site for RNA polymerase, while the operator is the region where the repressor can bind. In the absence of lactose, the repressor prevents transcription. In the presence of lactose, the repressor is unable to block the operator, allowing transcription of the structural genes.

Human Genome Project

The Human Genome Project (HGP) focused on sequencing the human genome and identifying and analysing its genes and genomic information.

The transcript discusses the human genome as containing approximately 3.16 billion bases, with around 20,000–25,000 genes. The average gene size is about 3,000 base pairs, while the dystrophin gene extends across approximately 2.4 million base pairs.

Major findings include:

• Chromosome 1 contains the highest number of genes.

• The Y chromosome contains comparatively fewer genes.

• Less than 2% of DNA is involved in protein coding.

• A large part of the genome consists of repetitive or non-coding DNA.

• Approximately 99.9% of DNA is shared between individuals, while remaining variations contribute to differences.

• The functions of more than half of the genes discussed in the lecture were unknown.

DNA Fingerprinting and VNTR

DNA fingerprinting, or DNA profiling, uses variations in DNA sequences to distinguish between individuals. An important basis of this technique is Variable Number Tandem Repeats (VNTRs).

VNTRs contain repeated DNA sequences whose number can vary between individuals. These differences create characteristic DNA patterns that can be compared between biological samples.

Satellite DNA is repetitive DNA and includes microsatellites and minisatellites.

The basic DNA fingerprinting procedure involves:

DNA isolation → DNA fragmentation → separation → transfer to membrane → probe hybridisation → detection

In this process, DNA is fragmented and separated according to size. The fragments are transferred to a membrane, and a labelled probe identifies the relevant VNTR sequences. The resulting bands can be visualised using autoradiography and compared between samples.

The Molecular Basis of Inheritance explains how genetic information is organised, maintained, expressed, and transmitted. The chapter covers the structure of DNA and RNA, DNA packaging, experiments establishing DNA as genetic material, semi-conservative replication, transcription, translation, and gene regulation.

The Human Genome Project further expanded understanding of the human genome, while repetitive DNA and VNTR variation provide the basis for DNA fingerprinting. Together, these concepts explain the molecular mechanisms through which genetic information is stored and expressed.